Subclinical hyperthyroidism is one of several causes of low serum thyroid-stimulating hormone (TSH) concentrations. It is most simply denned as low serum TSH and normal serum thyroid hormone concentrations. Other causes of low serum TSH concentrations from which subclinical hyperthyroidism must be distinguished are nonthyroidal illness and secondary hypothyroidism. The most common cause of subclinical hyperthyroidism is excessive thyroid hormone therapy. Other causes are an autonomously functioning thyroid adenoma, multinodular goiter, and Graves' disease. Because many patients have no evident thyroid disease and because serum TSH concentrations may be normal weeks or more later, intervention should not be seriously considered until the biochemical findings are found to persist. The likelihood of progression to overt hyperthyroidism is low. Some patients have minor symptoms and signs of hyperthyroidism, but most do not. Some may also have minor stimulation of cardiac function, an increased risk of atrial fibrillation, or accelerated bone loss. Management should be based on the cause of the subclinical hyperthyroidism and on whether the patient has any clinical manifestations of thyroid hormone excess or underlying problems likely to be worsened by small increases in thyroid secretion.